Conventionally, for example, in the image forming apparatus using the electrophotographic type, a surface of an electrophotographic photosensitive member (photosensitive member) as a member-to-be-charged is electrically charged by a charger, and thereafter the charged surface of the photosensitive member is exposed to light depending on image information, so that an electrostatic latent image is formed on the surface of the photosensitive member. As the charger for charging the surface of the photosensitive member, a corona charger (discharger) has been widely used.
In general, the corona charger is provided with supporting blocks at respective end portions of a shield case constituting a supporting casing which opens at one surface, and a discharge wire constituted by a wire material made of a metal such as tungsten is extended and stretched between these supporting blocks. In the case where a surface-to-be-charged of the member-to-be-charged such as the photosensitive member is charged using the corona charger, an open surface of the shield case is caused to oppose the surface-to-be-charged in a close state, and corona discharge is generated by supplying a discharge current to the discharge wire, so that electric charges are imparted to the surface-to-be-charged. At the open surface of the shield case, a grid is provided between the discharge wire and the surface-to-be-charged and a grid bias is applied to the grid to adjust an electric charge amount, so that also control of a charge potential of the surface-to-be-charged has been effected.
When a contaminant such as a toner, an external additive or paper powder or the like is deposited on the grid of the corona charger, non-uniformity (charging spot) generates in a potential distribution with respect to a longitudinal direction (longitudinal direction of the corona charger) of the surface-to-be-charged, so that an image defect generates in some cases. Therefore, the corona charger is provided with a cleaning member for cleaning the grid in some cases. For example, by moving the cleaning member along the shield case in a longitudinal direction of the corona charger (reciprocating motion or the like), removal of the contaminant deposited on the grid has been made.
Incidentally, in the image forming apparatus, in the case where an image forming unit is detachably mountable to an apparatus main assembly or in the like case, it is required that the image forming unit is stably supported by the apparatus main assembly in a mounted state. When this supporting state is unstable, for example, the image defect which is called banding generates in some cases. The banding is periodical density non-uniformity, with respect to a sub-scan direction, generating on an image. This is caused by generation of a deviation in image writing position or image transfer position by vibration of the image forming unit including the photosensitive member. Therefore, Japanese Laid-Open Patent Application Hei 11-316537 discloses that the vibration is prevented by providing an urging member between the image forming units in order to suppress the banding.
Further, also in the case where not only the corona charger is used as the charger for charging the surface of the photosensitive member for example, but also the corona charger itself is detachably mountable to the apparatus main assembly, the density non-uniformity generates on the image in some cases depending on a supporting state of the corona charger. Specifically, this is because a supporting member such as rails for supporting the corona charger are provided adjacently to respective sides of the corona charger with respect to a short direction, and in the case where the corona charger is supported by this supporting member, vibration from a vibration generating source in the apparatus main assembly is transmitted to the corona charger via the supporting member to fluctuate a distance between the photosensitive member and the corona charger and thus potential non-uniformity generates. Therefore, also with respect to the corona charger, it would be considered that the vibration is suppressed by a means such as contact of the supporting member, as disclosed in Japanese Laid-Open Patent Application Hei 11-316537, between the supporting members disposed on the respective sides of the corona charger with respect to the circumferential direction of the photosensitive member.